net: sockets: add support for SO_REUSEADDR

This commit adds support for the SO_REUSEADDR option to be enabled for
a socket using setsockopt(). With this option, it is possible to bind
multiple sockets to the same local IP address / port combination, when
one of the IP address is unspecified (ANY_ADDR).

The implementation strictly follows the BSD implementation and tries to
follow the Linux implementation as close as possible. However, there is
one limitation: for client sockets, the Linux implementation of
SO_REUSEADDR behaves exactly like the one for SO_REUSEPORT and enables
multiple sockets to have exactly the same specific IP address / port
combination. This behavior is not possible with this implementation, as
there is no trivial way to identify a socket to be a client socket
during the bind() call. For this behavior, one has to use the
SO_REUSEPORT option in Zephyr.

There is also a new Kconfig to control this feature similar to other
socket options: CONFIG_NET_CONTEXT_REUSEADDR. This option is enabled by
default if TCP or UDP are enabled. However, it can still be disabled
explicitly.

Signed-off-by: Tobias Frauenschläger <t.frauenschlaeger@me.com>